The present invention relates to laser scanning devices and, more particularly, to a laser scanning unit, a collimation assembly for use in a laser scanning unit and methods and apparatus for aligning the optical elements of one or more laser diode/pre-scan optical assemblies of a laser scanning unit.
Optical systems used in laser printers may be characterized as having three sub-systems or assemblies, namely, a laser diode/pre-scan optical assembly, a scanning assembly, and a post-scan assembly. Typically, the laser diode/pre-scan optical assembly includes a laser diode emitting a diverging laser beam, a collimator lens for collimating the beam emitted by the laser diode, and a pre-scan lens to focus the beam to a waist near the scanning device so that the post-scan assembly images the beam to a waist at a corresponding photoconductive (PC) drum surface.
The scanning assembly generally includes a scanning device such as a motor driven, rotatable polygon mirror having a plurality of peripheral mirror surfaces or facets that rotate during operation of the printer. The mirror surfaces reflect the collimated and focused beam received from the laser diode/pre-scan optical assembly. The direction of rotation of the polygon mirror determines the scan direction of the beam passing along a scanned object, such as a PC drum in a laser printer.
During a calibration procedure, the collimator lens must be properly aligned so that the beam exiting the collimator lens is properly position in process and scan directions. Further, a pre-scan lens must be properly positioned so that the laser beam waist is substantially located in the process direction, transverse to the scan direction, at the surface of a corresponding PC drum.
In a known laser diode/pre-scan optical assembly for a laser scanning unit, supporting structure is provided for mounting pre-scan lens structure comprising a pre-scan lens, and a laser diode/collimation assembly structure comprising laser diode and a collimator lens. The supporting structure is formed with precisely located mounting points held to close tolerances which accurately align the pre-scan lens structure and laser diode/collimation assembly structure to produce a laser beam output spatially oriented to a predetermined location and including the formation of a beam waist at a desired point relative to a corresponding PC drum surface. A certain degree of alignment of the pre-scan lens is provided by taking a physical measurement of the position of the pre-scan lens or its carrier relative to a reference feature formed on the supporting structure. While such an alignment procedure provides relatively accurate alignment of the pre-scan lens, it does not compensate for variations in the beam resulting from variations in the physical characteristics of the pre-scan lens, such as minor variations in thickness or aberrations in the pre-scan lens which may affect the character of the beam passing therethrough. Typically, the collimation assembly is merely secured to the housing without any alignment steps being performed. However, a known alignment procedure, performed as a repair operation, involves directing a video camera to a downstream optical component, such as a mechanical target, so that the location of the beam spot can be visually observed by an operator on a monitor. The operator then installs shims as necessary to the mounting points for the collimation assembly to locate the beam at a desired observed location on the mechanical target.
Accordingly, there remains a need for an optical system for a laser scanning unit in which the optical components of one or more of the laser diode/pre-scan optical assemblies may be readily aligned. In particular, there is a need for methods and alignment structure for aligning the optical components of each laser diode/pre-scan optical assembly in which the location of the beam produced by the laser diode/pre-scan optical assembly may be accurately determined and in which the optical components comprising the laser diode/pre-scan optical assembly may be readily adjusted.
A first aspect of the invention will now be described. In a laser scanning unit comprising a housing, and a laser diode for generating a laser beam, a collimation assembly comprising: a collimator lens; a collimation structure for supporting the collimator lens and an adjustable element provided for moving the collimation structure relative to the housing. The structure is adjustable relative to the housing so as to reposition the laser beam in first and second directions which are substantially perpendicular to one another.
The collimation structure is movable about a first axis which is substantially parallel to a scanning axis about which a scanning device adapted to be mounted in the housing is pivotally movable. Adjustment of the collimation structure about the first axis causes the beam to move in the first direction defining a scan direction. The collimation structure is movable about a second axis which is substantially perpendicular to the scanning axis and adjustment of the collimation structure about the second axis causes the beam to move in the second direction defining a process direction.
The collimation structure includes a first opening for receiving a pin defining a first axis about which the collimation structure may move for repositioning the beam in the first direction. The pin may comprise fixture structure. The fixture structure may comprise a portion of the housing or separate structure from the housing which is insertable into an opening provided in the housing.
The adjustable element may comprise a spring-biased spacer which preferably biases the collimation structure in a direction away from the housing so as to allow the beam to be repositioned in the second direction. The spring-biased spacer may comprise a belleville washer. It is also contemplated that the adjustable element may comprise a jack screw.
A second aspect of the present invention is as follows. In a laser scanning unit comprising a housing, a first laser diode for generating a laser beam, and a first collimation assembly for substantially collimating the laser beam and being movable relative to the housing, an alignment structure is provided for aligning the collimation assembly within the housing comprising: an alignment mount capable of being positioned adjacent to the housing; and an alignment sensor mounting to the alignment mount. The alignment sensor provides an indication of the location of a laser beam generated by the laser diode and passing through the collimation assembly in first and second directions. The first direction may be substantially perpendicular to the second direction.
The alignment mount may extend through an opening in the housing and is located along a line extending from the collimation assembly to said opening. The alignment sensor may comprise a quadcell sensor for sensing the location of the beam along two perpendicular axes.
The alignment structure may include a second laser diode for generating a second laser beam, and a second collimation assembly for substantially collimating the second laser beam and being movable relative to the housing. The alignment sensor further provides an indication of the location of the second laser beam.
In accordance with a third aspect of the present invention, a method is provided for aligning a collimation structure in a laser scanning unit comprising a housing, a first laser diode for generating a first laser beam, a first collimation assembly including the first collimation structure and a first collimator lens supported by the first collimation structure for substantially collimating the first laser beam, the method comprises the steps of: providing a sensor for detecting the position of a laser beam originating from the first laser diode and collimated by the first collimator lens, the sensor sensing the position of the beam in first and second directions; and adjusting the position of the first collimation structure to locate the laser beam at a predetermined location on the sensor.
The step of adjusting may comprise causing the laser beam to be repositioned in at least one of the first and second directions.
The laser scanning unit housing is adapted to receive a scanning assembly including a scanning device and the step of adjusting is preferably performed prior to the scanning assembly being located in the housing.
The scanning unit may further comprise a second laser diode for generating a second laser beam, a second collimation assembly including a second collimation structure and a second collimator lens supported by the second collimation structure for substantially collimating the second laser beam. The sensor may sense the position of the second laser beam. The method may further comprise the step of adjusting the position of the second collimation structure to locate the second laser beam at a predetermined location on the sensor.
A fourth aspect of the present invention will now be described. In a laser scanning unit comprising a housing, a scanning assembly including a scanning device, a laser diode for generating a laser beam, a collimation assembly for substantially collimating the laser beam and a pre-scan structure comprising a pre-scan lens carrier including a pre-scan lens supported for movement relative to the housing, an alignment structure for aligning the pre-scan lens carrier comprising: a beam splitter located along the path of a laser beam originating from the laser diode and passing through the collimation assembly and the pre-scan lens, the beam splitter forming the laser beam into first and second beam portions; and first and second sensors for sensing a spot size of each of the first and second beam portions, a desired position for the pre-scan lens carrier is determined based on the spot sizes (i.e., cross-section areas of the first and second beam portions) of the first and second beam portions.
The alignment structure may further comprise a processor for determining a location of a waist of the laser beam relative to a known location of a surface of a corresponding photoconductive member based on the spot sizes of the first and second beam portions.
In accordance with a fifth aspect of the present invention, a method is provided for adjusting a pre-scan lens in a laser scanning unit comprising a housing, a scanning assembly including a scanning device, a laser diode for generating a laser beam, a collimation assembly for substantially collimating the laser beam and a pre-scan structure comprising the pre-scan lens which is supported for movement relative to the housing, the method comprises the step of positioning a beam splitter in the path of a laser beam originating from the laser diode and passing through the collimation assembly and the pre-scan lens. The beam splitter forms the laser beam into first and second beam portions. The method further comprises the steps of providing first and second spaced-apart sensors for sensing the spot size of each of the first and second beam portions and moving the pre-scan lens relative to the housing to obtain a laser beam waist at a predetermined location.
The method may further comprise the step of determining a location of the waist of the laser beam relative to a known location of a surface of a corresponding photoconductive member based on the sensed spot sizes of the first and second beam portions.